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Carbon-monoxide Lasers Disrupt Current Micro-via Drilling Technology

Keywords: Laser processing, µ-via drilling, Carbon-monoxide laser

While mechanical drilling of micro-vias has long been established, the use of laser-based micro-via drilling tools is growing at twice as fast than mechanical drilling. Shrinking via diameters have tipped the scale in favor of laser-based drilling tools.
CO2 lasers have long been the work horse for drilling vias around 100µm diameter. CO2 lasers drill vias through a thermal interaction. The material absorbs the infrared light from the CO2 laser, which heats it until it vaporizes. While the CO2 laser can readily produce a smaller via than a mechanical drill, there are limitations on the smallest via diameter it can reach. One limit is caused by light diffraction. The smallest focused spot size to which a laser beam can be focused is directly related to its wavelength. The CO2 lasers 10µm wavelength, can’t be focused as small as visible or ultraviolet wavelengths. As a result, the practical lower limit on via diameters for CO2 lasers is about 70 µm.
Carbon monoxide (CO) lasers allow to achieve smaller vias. First researched decades ago, lifetime and reliability issues continued to prevent commercial use - until 2015. In the last months CO lasers with output over the 5 µm to 6 µm spectral range, which is about half the CO2 wavelength and hence allows for a smaller focus spot, have become commercially available.
CO lasers can achieve via diameter down to 35 µm at the same cost of ownership as CO2 lasers. But, even when producing larger diameter vias, the CO laser has an edge over CO2. Specifically, the focusing lens used to achieve a 70 µm diameter via with a CO laser has twice the focal length of the lens required to achieve the same via size with a CO2 laser. This longer focal lens provides greater depth of focus, which increases the field of view. The longer focal length and increased depth of field facilitate an increase in scanning speed, and therefore faster via production, with the shorter wavelength CO laser.
Because the CO laser can be focused to a smaller spot, it’s easier to reach higher power densities with it than with a longer wavelength CO2 laser of the same power. This higher fluence (intensity) enables it to more readily drill untreated copper. Conversely, achieving a given power density requires only one fourth the total output power with a CO laser as with a CO2 laser. Depending upon the exact parameters of a particular via drilling task, this makes it possible to use a much lower power CO laser for a specific job.
In addition, CO lasers show much better quality results when machining polyimide, which opens a new application space for the use of polyimide in advanced packaging.
This paper will compare and contrast various via drilling methods and explain at which is best to use depending on hole diameter, hole density and material.